1. Field of Invention
This invention relates generally to thermal printers having a transport system for moving a receiver medium and a dye-donor web past a thermal print head to transfer a dye image to the receiver medium; and more particularly to an improved media guide for such printers.
2. Background
Small platen thermal printers use a print head with individually addressable heating elements to transfer dye from a donor to a receiver. Color images are produced by printing one dye color on the receiver and then rewinding the receiver prior to printing another dye color. When all dye colors are printed, the completed print is ejected.
Referring to FIGS. 1 and 2, a prior art thermal printer 10 includes a print head assembly 12 and dye-donor web supply and take-up spools 14 and 16, respectively. The printer includes a roller platen assembly 18, a pair of dye receiver medium transport mechanism pinch rollers 20 and 22, and a dye receiver medium supply 24.
Normal thermal printer operations include loading dye receiver medium, printing information upon the dye receiver medium and ejecting the finished print. Each of these operations is fully described in commonly-assigned U.S. Pat. No. 5,176,458, which issued to H. G. Wirth on Jan. 5, 1993. The disclosure of that patent is hereby incorporated into this specification by reference.
Printer operation begins with a loading phase, in which print head assembly 12 moves to a loading position. A dye-donor web 26 and a sheet 28 of dye receiver medium advance along converging paths to a printing location, and print head assembly 12 is positioned in preparation for the printing operation.
As a sheet 28 of dye receiver medium advances, it moves along a guide 30 to follow a curved path toward a gap between print head assembly 12 and platen assembly 18. As the dye receiver medium moves into this gap, it contacts dye-donor web 26 and is guided toward dye receiver medium transport mechanism pinch rollers 20 and 22.
Once dye receiver medium 28 is firmly held by dye receiver medium transport mechanism pinch rollers 20 and 22, print head assembly 12 moves toward platen assembly 18, pressing dye-donor web 26 and dye receiver medium 28 against platen assembly 18 to form a sandwich for thermal printing.
When the loading phase is completed, printer 10 enters a printing phase, during which print head assembly 12 presses dye-donor web 26 and dye receiver medium 28 into platen assembly 18, and prints information on the dye receiver medium.
FIG. 2 is an enlarged view of the print head and roller platen assembly region of the prior art thermal printer shown in FIG. 1. Donor web 26 and receiver sheet 28 are sandwiched between print head assembly 12 and roller platen assembly 18 during printing. Receiver sheet guide 30 directs receiver sheet 28 toward the print head. Mechanical interference constraints on this design require receiver sheet guide 30 to end some distance from the nip between the print head and the roller platen assembly. To insure that receiver sheet 28 moves properly in a receiver media transport path, a portion of the receiver sheet must remain between guide 30 and platen assembly 18 during the entire printing process. Thus, the distance from the end of receiver sheet guide 30 to the nip represents the smallest trailing border that can be provided on a print in the print direction.
Print heads in thermal printers typically include electronics packages which must be protected from damage. Still referring to FIG. 2, a standard print head electronics package cover 32 is provided. Dye donor web 26 from the supply spool passes over a separate upstream donor guide 34 and across electronics package cover 32 before passing through the nip between the print head and the roller platen assembly. Dye donor web 26 and dye receiver sheet 28 move downstream from the nip, still pressed together, until they reach a donor-receiver separation guide 36. From this point, dye donor web 26 separates from dye receiver sheet 28, with the dye donor web continuing on to take-up spool 16 (FIG. 1) and dye receiver sheet 28 being delivered to an output tray 38 (also FIG. 1).
This typical thermal printer configuration has two problems. First, it restricts the minimum border dimension in the print direction to at least the distance between the end of receiver sheet guide 30 and the nip between print head assembly 12 and roller platen assembly 18. The second problem is that scratches can be produced by the end of receiver sheet guide 30 rubbing on the image surface of receiver sheets 28.